Monarchs Need Moisture
Moisture is crucial for the butterflies' overwintering success in Mexico. I hypothesize that one of the reasons the butterflies choose to overwinter where they do is because the high altitude mountains capture moisture. The pictures in this slideshow tell the story. I took the picture below early in the morning, as the clouds were forming and blowing through the butterfly colony. (The butterfly colony is just outside of the image, to the left.)

Early in the morning, the mist is engulfing the butterflies and condensing on the Oyamel fir trees.

The mist freezes into frost.
Taken early in the morning on 11 December 2006, this picture shows how the condensed mist freezes on the magnificent open area on Cerro Pelon, known as the Llanos de los Tres Goberonores. In this picture, you cannot see the colony, but it is all the way down at the end, to the right, in a grove of cedar trees.

Thirsty monarchs drink dew (melted frost).
Later in the morning, in the same area, we found several thousand monarchs drinking dew. The sun's rays had fallen on the frosted ground vegetation and the frost had melted into dew. The monarchs flew out of the overwintering colony to drink the dew drops.

Here is a monarch with his proboscis in the grass, imbibing the dew.

Water droplets condensed on the monarch's wings.
This monarch failed to fly back into the clusters before nightfall. Water droplets condensed on its wings. This is a very precarious situation. The dew can freeze when the overnight temperature drop below freezing. This often happens in open areas that are exposed to the cold, clear sky.

Some monarchs are killed by frost on their wings.
This butterfly is a victim of one of these freezing events and will not survive. The frost crystals mortally wounded the butterfly. They penetrated through the cuticle and disrupted the cells. Although millions of monarchs may fly out of the colony on sunny days to find water to drink, almost all of them manage to fly back into their colonies. Only a few get stranded in open areas where they may freeze to death.

How the Mountains Capture Moisture
The high altitude mountains capture moisture through the process of adiabatic condensation. This process occurs when moisture-laden winds blow up and over the mountains.

Clouds Form as Air Rises
As air rises, it expands and cools. Then clouds form. This is called adiabatic condensation.

Mist Condenses on the Trees and the Butterflies
At the high altitudes, the moisture condenses as water droplets on the needles of the Oyamels and pines. You can see droplets of condensed mist on both the pine needles and on the butterflies.